ORION NanoFab: An Overview of Applications White Paper
ORION NanoFab: An Overview of Applications Author: Dr. Bipin Singh Carl Zeiss NTS, LLC, USA Date: September 2012 Introduction With the advancement of scientific research, nanofabrication is continually being pushed to its limits. For rapid prototyping and fast turnaround, gallium focused ion beam (FIB) based instruments are often used. These charged particle instruments offer the greatest flexibility and the highest throughput for prototyping customized devices for research and development as well as industrial applications. A new entrant in the field of nanofabrication is the ORION NanoFab, an inert ion species based focused ion beam instrument made by Carl Zeiss. Just like a gallium FIB, helium and neon ions can be used for nanofabrication. However, unlike gallium, features made with helium and neon can go down to few nanometers. In addition, the helium ion beam can be used to generate high resolution images of the samples in the same instrument. In this white paper, an overview of applications enabled by this instrument will be provided. The helium or neon focused ion beam is then typically rastered across a sample surface using beam steering electronics to either image the sample or to fabricate structures and devices. Multi-Ion Beams of the ORION NanoFab The ORION NanoFab is the only tool of its kind. It advances the class-leading GFIS technology and features a single column dual beam tool that can generate helium or neon focused ion beam (one at a time) in the same GFIS column. In addition, an optional gallium FIB column can be integrated to ORION NanoFab. Thus, ORION NanoFab is the first commercially available multi-ion beam tool featuring helium, neon and gallium FIB capabilities (Figure 1). ORION NanoFab: The Principles of Operation ORION NanoFab is a third generation gas field ion source (GFIS) based machine by Carl Zeiss that offers a selection of helium and neon focused ion beam. The details about the GFIS technology can be found elsewhere. 1 Briefly, a cryogenically cooled sharpened metal tip that is positively biased with respect to a grounded counter electrode is exposed to very low quantities of helium (or neon) under vacuum. A proprietary method is used to modify the metallic tip in situ such that only three atoms remain at the end of the tip. These three atoms are referred to as the trimer. Under large positive bias, the trimer emits three streams of helium (or neon) ions. One of these ion streams is then aligned and focused by the column optics and becomes the helium or neon focused ion beam. Figure 1 A schematic representation of multi-ion beam concept on ORION NanoFab 2
Advantages of Ions Over Electrons for Nanofabrication A variety of methods can be used for fabricating devices and prototypes at the nanoscale. A summary of the methods for nanofabrication using electron and focused ion beams can be found elsewhere. 2 The electron beam based systems are used for nanofabrication by mostly the following two methods: (1) electron beam lithography or (2) electron beam induced deposition or etch in presence of precursor reactive gases. Due to their low mass, electrons cannot be used for direct sputtering of materials. Of course at very high voltages (several hundred kv), electron beams gain high enough energy to punch holes in thin membranes and or modify surfaces or materials. However, this process of making features with high energy electrons is time-consuming and hard to control. Electron beam lithography (EBL) involves exposing the photoresist with an electron beam and then depending on the nature of the photoresist (positive or negative tone), the exposed region is either retained or dissolved during the development process. This method is used in many applications such as writing of structures on photomasks for lithography, hard drive manufacturing and prototyping of many types of devices for research and development. Another way in which electron beam can be used for nanofabrication is by using in conjunction with a gas injection system. In a gas injection system, a precursor gas is introduced in a scanning electron microscope system and the electron beam is scanned on a sample. Depending on the precursor gas used, conductive or insulating materials can be selectively deposited in areas that are exposed by electron beam. If a precursor gas such as xenon difluoride is used, substrates can be etched aggressively, again selectively only in areas that are exposed by the electron beam. More details of such systems can be found elsewhere. 3 When considering ions, there are some unique advantages to nanofabrication. With electons, you can only manipulate their energy and current. However, in a machine like ORION NanoFab, in addition to energy and current, the ion species can also be chosen to fit the job to be done. The specific advantages of light ions such as helium will be described in the next section, but it would suffice to state here that by having a choice of ion species, one can perform massive material removal with, let s say, gallium FIB and do very fine milling with helium ion beam. This method of nanofabrication is also known as ion beam milling and the underlying phenomenon is called sputtering. The lithographic patterning with gallium FIB cannot compete with electron beam lithography in the terms of feature-size. However, when one uses helium and neon ions to expose photoresist, patterning is possible at the same length scale as what can be achieved with electron beam lithography. 4,5 In fact, helium and neon are a superior ion species when it comes to lithography. Sub-10 nm dots and lines can be routinely made with helium and neon ion lithography. In sub-10 nm lithography, electron beam lithography suffers significantly from proximity effect. A distinct advantage of helium and neon ion beam lithography is that there is no proximity effect. This means that when fabricating nested structures, the width of features in the center and periphery of the region of interest will be the same. The user can then focus on designing and fabricating nanostructures of choice rather than engineering elaborate dose adjustments to overcome proximity effect. Advantages of Helium and Neon Over Gallium Ions for Nanofabrication Gallium FIB is a tool of choice for rapid-prototyping applications. Gallium FIBs are routinely used to cross-section samples for failure analysis of semiconductor chips and other types of samples, sample preparation for transmission electron microscopy (TEM), mask repair, and integrated chip modification. While great at milling structures larger than 30 nm in size, Ga FIB is not suited for sub-30 nm regime for nanofabrication. Although the probe size can be as low as ca. 2 nm, the smallest feature size made using Ga FIB is rarely below 30 nm due to interaction with the sample that is milled. Neon ions, on the other hand, are lighter than gallium ions and sputter away material at a smaller scale. Helium ions are even lighter and can be used to make very fine nanostructures. The sizes of structures that can be made with neon and helium ions by milling can routinely reach sub-10 nm length scales. A computer software (SRIM) can be used to understand the scattering effects of ion beam in samples. Table 1 compares the sputter yield of 30 kev gallium, neon and helium ions which can give a sense of milling rates with these three ion species. While helium ions cannot be used for ion milling of very large volumes of material, neon ions are quite effective at removing larger quantities of material. For example, Figure 2 shows image of aluminum posts that were cut using a focused neon ion beam. 3
When used with a gas injection system (GIS), helium and neon ions deposit finer features than what is possible with gallium ions. The metals deposits are higher quality as they don t have any gallium contamination. In addition, the insulator deposits are of better quality and exhibit higher resistivity as they don t have gallium in the deposit. Imaging Capabilities of ORION NanoFab In addition to the nanofabrication capabilities, ORION NanoFab also offers high resolution imaging. The imaging capabilities of the ORION platform are well-regarded by the scientific community and has resulted in more than 100 scientific publications. Table 1 Comparing the incident ions interactions in aluminum and gold samples. Data derived from the SRIM simulation of 10,000 incident ions at normal incidence. Incident Beam Aluminum Sample Sputter Yield 30 kev He 0.06 0.153 30 kev Ne 1.78 4.39 30 kev Ga 3.90 17.4 Gold Sample Sputter Yield There are three attributes to imaging with ORION NanoFab that should be emphasized (1) Imaging of insulating samples ORION NanoFab excels at imaging uncoated insulating samples. Unlike in a scanning electron microscope, the samples accumulate only a positive charge in an ORION NanoFab which is very elegantly neutralized by a built-in electron flood gun. This means that one doesn t need to coat the samples with conductive over coat in order to get nanometerresolution images. (2) Large depth of field The images generated by ORION NanoFab have 5-10 times depth of field as compared to those generated by a high end FE-SEM. If you have a highly three-dimensional sample, this will result in an image that shows all high and low points of the sample in great detail and focus. (3) High resolution surface-sensitive imaging The images generated by ORION NanoFab are sub-nm in resolution which results in immense details in the images that are captured. Due to the interaction physics of helium ion beam and sample, it turns out that secondary electrons are generated from the sample from the top 5 nm only. In addition, these secondary electrons are SE2 only. This results in immense surface sensitivity in the images. Surface details at high resolution can be seen by ORION NanoFab. Figure 2 Milling capabilities on ORION NanoFab demonstrated by cutting of aluminum posts 4
ORION NanoFab White Papers The following white papers are available from Carl Zeiss which highlight key capabilities of ORION NanoFab: White Paper: Nano-pore milling This white paper describes how ORION NanoFab can help in fabrication of pores with diameter smaller than 10 nm. White Paper: Helium and Neon Ion Beam Lithography used for helium and neon ion beam lithography. White Paper: Graphene Nano-ribbon patterning used for making large aspect ratio graphene nano-ribbons in single step. Summary ORION NanoFab is the world s first helium ion microscope focused ion beam instrument offered by Carl Zeiss. Built upon the advancement in the class-leading Gas Field Ion Source (GFIS) technology, ORION NanoFab provides a complete sub-10 nanometer nanofabrication and sub-nanometer imaging solution for the industry, government, and academic research laboratories. ORION NanoFab is the world's first instrument to incorporate neon and helium ion beams in single GFIS column with an optional gallium focused ion beam (FIB) column. The neon ion beam offers outstanding machining and nanofabrication capabilities due to higher sputter yields (for ion beam milling) and faster resist exposure (for ion beam lithography). The helium ion beam allows sub-10 nm nanofabrication as well as high resolution imaging capability in the same instrument. The addition of an optional state-of-the-art gallium FIB column makes the ORION NanoFab the most versatile nanofabrication and imaging platform in the world. White Paper: Fabrication of Solid-State Nano-pores for Biomolecule Detection used for making functional DNA sequencing devices. White Paper: Sub-10 nm Nano-machining with Multiple Ion Beams. used for fabrication of a double bow tie plasmonic device using gallium, neon and helium ion beam. White Paper: Plasmonic Device Fabrication used for fabrication of plasmonic device with sub-10 nm gaps. White Paper: Advancing Oil and Gas Exploration used for getting high resolution images of organic porosity and network of nano-pores in shale rock and coal. References: 1 N. P. Economou, J. A. Notte and W. B. Thompson, Scanning 33, 1-7 (2011). 2 Nanofabrication Using Focused Ion and Electron Beams, edited by I. V. Utke et al., Oxford University Press, 2012. 3 I. Utke, P. Hoffman and J. Melngailis, Journal of Vacuum Science and Technology B 26, 1197-1276 (2008). 4 D. Maas et al., Proceedings of SPIE 7638, 763814 (2010) 5 D. Winston et al., Nano Letters 11, 4343-4347 (2011). 5
facebook.com/zeissmicroscopy twitter.com/zeiss_micro youtube.com/zeissmicroscopy flickr.com/zeissmicro 42_011_017 CZ-09/2012 Design, scope of delivery and technical progress subject to change without notice. Carl Zeiss Microscopy GmbH Carl Zeiss Microscopy GmbH 07745 Jena, Germany microscopy@zeiss.com www.zeiss.com/microscopy